Where the delicate and noninvasive mixing of small-sized fluid samples may be called for, one known technique is to use a rotating magnetic field to mix the fluid. Typically, magnetically inducible particles, such as paramagnetic microspheres, are suspended in the fluid to be mixed. The resulting particle suspension is then placed in a close proximity to a magnetic field such that the flux lines of the magnetic field pass through the suspension in one direction, and substantially in parallel.
FIG. 1 shows a diagram depicting a conventional coil arrangement that mixes the suspended magnetically inducible particles by rotating them using a unidirectional magnetic field. The unidirectional magnetic field is generated electromagnetically using a set of Helmholtz coils 201a, 201b. Each Helmholtz coil set 201a, 201b consists of two wound coils 201a, 201b wired in series, and arranged along a common coil axis. When electrical power is applied to the coils 201a, 201b, a uniform, unidirectional magnetic field is produced. The strength of this magnetic field is proportional to the number of turns that are present in the coils 201a, 201b, the applied electric current, the physical size of the coils, and the spacing between the coils. Suspended magnetically inducible particles 204 positioned between and along the common axis of coils 201a, 201b experience the uniform and unidirectional magnetic field.
FIG. 2 depicts suspended magnetically inducible particles that are subjected to a unidirectional magnetic field, such as the one generated by the conventional device shown in FIG. 1. As may be seen in FIG. 2, the suspended magnetically inducible particles tend to align themselves along the unidirectional magnetic field lines. As a result, long chains of particles 401 are formed, aligned in parallel and in the same direction through a particle suspension area 204.
To rotate these particle chains 401, at least one additional set of coils 203a, 203b is typically positioned such that its common coil axis is provided at a 90-degree angle from the common axis of the first set of coils 201a, 201b, as shown in FIG. 1. A 90-degree out-of-phase sinusoidal variation in power is then applied to each set of coils 201a, 201b and 203a, 203b, which produces a proportional variation in the strength of the magnetic fields H201 and H203 generated by coil sets 201a, 201b and 203a, 203b respectively. The particle chains 401 tend to align and realign themselves along the strongest lines of the magnetic flux. Varying the strength of the magnetic fields H201 and H203 produced by coil sets 201a, 201b and 203a, 203b respectively as described above, causes the unidirectional particle chains 401 to rotate substantially about their respective centers, thereby mixing the particle suspension area 204.
There are certain disadvantages to magnetic mixing devices that employ the above-described Helmholtz coil arrangement. One such disadvantage is the relative complexity of such devices, since the proper operation of the Hehnholtz coils requires the use of function generators, power amplifiers and cooling systems, among other things. Another disadvantage is that the mixing effect in the conventional Helmholtz coil-based system is substantially limited to the immediate area spanned by the rotating particle chains 401, each of which rotates about its own center. Thus, in order to spread the mixing effect throughout the particle suspension area 204, many particle chains 401 are needed. This makes an inefficient use of the available magnetically inducible particles in the particle suspension area 204.
A second conventional arrangement uses a disc-shaped strong rare-earth magnet in place of Helmholtz coils 201a, 201b and 203a, 203b. FIG. 3 shows such a conventional arrangement, which includes a disc-shaped magnet 301 that is mounted edge-wise on a motor shaft 302 that rotates the magnet 301 relative to the particle suspension area 204.
Similarly to the conventional Helmholtz coil-based arrangement described above, the conventional magnet-based mixing arrangement of FIG. 3 applies a unidirectional magnetic field to the suspended magnetically inducible particles in the area 204 contained in the fluid cell 202. Referring to FIG. 3, the edge of the magnet 301 is arranged with respect to the fluid cell 202 such that the magnetic axis 306 of the magnetic field generated by the magnet 301 is perpendicular to an axis of rotation 304 of the magnet 301. The magnetic flux lines produced by the magnet 301 extend approximately in parallel and unidirectionally through the fluid cell 202. The magnetically inducible particles align themselves along the unidirectional magnetic field lines in the long, unidirectional chains 401 (see FIG. 2). As the magnet 301 is rotated about the axis of rotation 304, the magnetic field produced by the magnet 301 also rotates, thus causing the particle chains 401 to rotate around their respective centers to mix the suspension area 204.
However, the described conventional magnet-based arrangement also has certain disadvantages. First, just as with the conventional Helmholtz coil arrangement, the particle chains 401 rotate about their respective centers. Thus, the mixing effect produced by rotation of the magnet 301 is still limited to the immediate area spanned by the rotating, unidirectional chains 401. Second, the farther the area 204 is from the magnetic axis 306 of the magnet 301, the weaker the magnetic field becomes, and the lesser the tendency of the particles to form themselves into chains. Since the mixing effect is a function of the length of each particle chain 401, the mixing effect produced by the rotation of the magnet 301 becomes progressively weaker the farther away the particles are located from the poles of the magnet 301.